1. Field of the Invention
This invention relates to a brake band apparatus for use in the automatic transmission of a vehicle, a construction machine, an agricultural machine or the like.
2. Related Background Art
A one-wrap brake band has heretofore been used in a brake band apparatus for fastening with a self-engaging force exerted on the rotation of a drum (inner ring) in both directions. FIG. 8 of the accompanying drawings is a side view showing a brake band apparatus 51 according to the prior art.
The brake band apparatus 51 is comprised of a rotatable drum 52, a substantially C-shaped brake band 61 disposed on the outer periphery of the drum 52, and two actuators 71 and 81 for giving a load to the brake band 61.
The brake band 61 is comprised of a band 62, a frictional material 63 secured to the inner peripheral surface of the band 62, and brackets 66 and 67 fixed to the opposite end portions of the band 62 which are free ends.
The two actuators 71 and 81 are identical and therefore, only the construction of one actuator 71 will be described. The actuator 71 is comprised of a substantially cylindrical case 75, an oil chamber 73 provided in the case 75, an oil supply hole 77 leading to the oil chamber 73, a hole 78 for letting pneumatic pressure created in a void 72 escape outwardly, a piston 74 sliding on the inner wall of the case 75, and a rod 79 provided integrally with the piston 74. Further, in the void 72, a return spring 80 is mounted around the rod 79.
The operation of this brake band apparatus is such that oil pressure is supplied to the oil chamber 73 through the oil supply hole 77 to thereby create a load to the brake band 61, and fasten the drum 52. On the other hand, to liberate the fastening state of the brake band 61, the oil pressure in the oil chamber 73 is drawn, and the piston 74 and the rod 79 are retracted by the biasing force of the return spring 80, and the band 62 is restored.
The operating state of the apparatus at this time is shown in FIG. 9 of the accompanying drawings. In FIG. 9, the rotational direction of the drum 52 is a counter-clockwise direction (indicated by arrow in drawings) and therefore, oil pressure is supplied to the oil chamber 73 of the actuator 71, the piston 74 is moved rightwardly as viewed in FIG. 9, and the rod 79 mounted on the piston 74 biases the bracket. The bracket 67 provided on the other end of the band 61 cannot be moved because a rod 89 abuts against it in a fixed state. By this construction, the brake band 61 fastens the drum 52.
The rod 89 supporting the bracket 67 may be completely fixed, but it has also been practised to support the bracket so that the rod 89 may deviate somewhat rightwardly as viewed in the figure so as to alleviate the shock to the band 62 during fastening in conformity with the coefficient of friction of the frictional material 63 which is fluctuated by the environment such as the temperature of lubricating oil.
On the other hand, when the rotational direction of the drum 52 is clockwise, the operations of the actuators 71 and 81 become opposite to what has been described above, as shown in FIG. 10. Also, the function of alleviating the above-mentioned shock occurring during fastening is likewise performed by the operation of the other actuator 71 when the rotation of the drum 52 is opposite in direction (the direction of arrow).
The direction of the above-described load to the brake band 61 is the direction in which the drum 52 is rotated (hereinafter referred to as the rotational direction side), and by the direction being made identical, the self-engaging force of the brake band 62 works and the drum 52 is fastened. The torque capacity of the brake band by this self-engaging force is represented by the following expression: EQU T=F.multidot.R.multidot.(e.sup..mu.B -1)
where T: torque, F: load (apply), R: the outer diameter of the drum, e: natural logarithm, .mu.: coefficient of friction, .beta.: angle of contact.
However, the above-described brake band apparatus according to the prior art is complicated in mechanism and control because two actuators need be provided. Also, the frictional material secured to the inner peripheral surface of the brake band must receive rotations in two directions and is high in frequency of use, and this has led to the problem that the life thereof becomes short.